Conventionally, a wheel loader is known as a work machine. In a wheel loader, an attachment such as a bucket or the like is provided at a distal end of a boom pivoted on a vehicle body, the boom is provided in a manner movable up and down by a boom cylinder, and the bucket is driven via a Z-shaped link.
As shown in FIG. 15, the Z-shaped link includes a bell crank 11 rotatably pivoted substantially on the central portion of the boom 10, a tilt cylinder (see dashed lines) connecting an upper end of the bell crank 11 and the vehicle body (not shown), and a connecting link 13 that connects a lower end of the bell crank 11 and a rear portion of the bucket 20.
Incidentally, in FIG. 15, the boom cylinder and the tilt cylinder are omitted to simplify the figure. A pivot region (pivoting region) Z of the tilt cylinder relative to the vehicle body is drawn on the boom 10 in the figure, but actually is disposed on the vehicle body (not shown), not on the boom 10. In FIG. 15, the bucket 20 at a ground position, an intermediate position, and an uppermost maximum height position is shown.
In such a wheel loader, a digging operation is carried out with the bucket 20 disposed near the ground position, and a loading operation is carried out such that a load is dumped onto a truck from the intermediate position or a top position. The dumping accompanies so-called “load leveling”, that is, leveling topside of earth and sand loaded on a dump truck or the like. The “load leveling”, in which a height of the bucket 20 is adjusted mostly via an operation of the boom cylinder, is not efficiently conducted unless an angle of a lower surface 21 of the bucket 20 is horizontal.
In order to start the digging operation immediately after having loaded earth and sand onto a dump truck or the like, the work machine is provided with a function called automatic leveler for activating the tilt cylinder to change the angle of the lower surface 21 of the bucket 20 to a horizontal state on the ground position without a manual operation by an operator. If the lower surface 21 of the bucket 20 in the maximum height position is greatly tilted downward toward a distal end thereof, the bucket 20 contacts with a loading space of the dump truck or the like when the wheel loader is receded, thereby blocking the receding movement of the wheel loader.
Further, if the distal end of the lower surface 21 of the bucket 20 at a maximum height thereof is tilted downward, the earth, sand, and the like drop onto the ground when the wheel loader is receded, which may cause troubles to the operations.
Therefore, an angle of the lower surface 21 of the bucket 20 when lifted from the ground level position to the maximum height position without operating the tilt cylinder is preferably as close to horizontal as possible.
Taking the above into consideration, angle characteristics of the attachment is improved in a wheel loader (disclosed in, e.g. Patent Document 1). According to such improvement, the bell crank 11 is tilted toward the attachment or not tilted at all when the bucket 20 is on the ground.
In addition, a wheel loader in which a fork is combined with the Z-shaped link is also known (e.g. Patent Document 2).
Patent Document 1: WO2005-012653
Patent document 2: JP-A-63-22499